The present invention relates to a system for controlling the fuel injection of an automotive engine in dependence on a throttle opening degree and engine speed.
In a known fuel injection system, a basic fuel injection pulse width Tp is calculated in dependence on throttle opening degree .theta. and engine speed N. The basic pulse width Tp are stored in a table shown in FIG. 9 and are derived for controlling the fuel injection during the operation of the engine. At a transient state of the operation of the engine, the basic fuel injection pulse width Tp is corrected in dependence on various factors such as engine speed, pressure in an intake passage, coolant temperature and vehicle speed, so that air-fuel mixture is prevented from becoming rich or lean (see for example, Japanese Pat. Laid Open Nos. 58-48720 and 58-41230).
However, since there is a space between the throttle valve and a cylinder of the engine, such as a chamber formed downstream of the throttle valve, changing of actual amount of inducted air per engine cycle in response to the change of the throttle opening degree during the transient state is delayed. Accordingly, when the throttle valve is rapidly opened, the air-fuel mixture becomes rich. To the contrary, when the throttle valve is rapidly closed, the air-fuel ratio becomes lean.
In order to overcome such a defect, it is preferable to estimate the quantity of the air inducted in the cylinder of the engine in one cycle and to correct the basic injection pulse width based on the estimated quantity.
Referring to FIGS. 2a and 2b, the intake system schematically illustrated in FIG. 2a approximately equals to an electric circuit of FIG. 2b. Namely, the pressure P in the intake passage 2 at downstream of the throttle valve 3 and chamber 5 corresponds to the voltage V and a quantity Q.sub..theta. corresponds to the current I.sub..theta. in FIG. 2b. Po represents a pressure at upstream of the throttle valve 3 and corresponds to the voltage Vo in FIG. 2b. A resistance R.sub..theta. corresponds to a resistance at the throttle valve 3 and a resistance Re corresponds to a resistance in the engine 1. In other words, R.sub..theta. is a variable determined by the throttle valve opening degree .theta., and Re is a variable determined by the engine speed N. The relationships among the voltages V, Vo and resistances R.sub..theta., Re can be expressed as the following equations. ##EQU1## where C is a constant for the capacity of the intake passage in downstream of the throttle valve and the throttle chamber 5, and .tau. is a time constant. It will be seen that the pressure P delays with respect to engine speed N and the throttle opening degree .theta. with a first order lag determined by the time constant .tau..
Assuming the basic injection pulse width Tp proportional to the pressure P, Tp=KP, where K is a constant dependent on volumetric efficiency. Namely, the basic injection pulse width Tp varies in accordance with the pressure P. Therefore, it is preferable that the basic injection pulse width Tp has a time lag of first order determined by the time constant .tau., with respect to a basic injection pulse width Tp* obtained from a table in accordance with throttle opening degree .theta. and engine speed N.